1. Field of the Invention
This invention relates generally to flow meters and, more particularly, to flow meters containing improved insulating materials and thermal shields which are useful in the sensitive measurement of the mass flow of a fluid which can be either a gas or a liquid.
2. Description of the Prior Art
In the past, mass flow meters have been developed and manufactured by various companies for the purpose of carefully measuring or sensing fluid flow. Many applications have been developed for the use of mass flow meters such as in deposition equipment to carefully control fluids, usually gases, for the formation of deposited layers on objects like semiconductor wafers in order to assist in the fabrication of semiconductor chips containing thousands of semiconductor devices and, in some cases, thousands of integrated circuits.
Thus, in fabricating, for example, semiconductor chips having thousands or tens of thousands of integrated circuits contained in each of the chips, it became much more difficult to control and increase the yields in making good chips. As the chips became more complex and packed with more devices and increased amounts of integrated circuits, the yield problems associated with producing good chips became greater and greater. As the technology has been increased towards making chips having more than a million integrated circuits such as million plus bit random access MOS type memory chips, the necessity of more precisely controlling all the different phases of chip manufacture became extremely important. Accordingly, a need existed to provide very sensitive and accurate mass flow devices to more carefully control the fluids used in the semiconductor fabrication process and thereby increase yields in making good semiconductor chips. Deposition materials such as polysilicon, which in many applications has to be carefully doped with certain impurities to provide a desired electrical resistance or conductivity, are often needed to manufacture complex semiconductor chips having large numbers of integrated circuits. Therefore, the importance of very accurate and sensitive flow meters to carefully control the thickness of the depositions and the mixing of impurities or materials to obtain certain desired electrical characteristics continues to become more evident as chips containing integrated circuits increase in circuit density.